When I turn against a company I don't go back. It isn't political or a boycott but I haven't been to KFC since the Oprah free chicken debacle. There are companies I have been boycotting for political reasons for less time but I don't think I'll be back for at least decades.
You vastly overestimate the resolve of most consumers.
It's probably closer to 95% of people who give up on their boycott once they realize just how enmeshed company is in their market and how much extra effort it takes to avoid giving them any money.
I haven't faltered in my boycott of nestle. It's easy to avoid all of their products (and their sub brands), as there are plenty of alternatives.
But you're right. Literally impossible to destroy them with boycotting alone, since the many many businesses who do business with them. My solo boycott is barely making a dent.
It's very easy when there's so many better chicken sandwich alternatives. Popeyes chicken for example, Wendy's. Not to mention the half a dozen local joints.